This invention relates to an autoclear circuit included in a large scale integration (abbreviated as LSI) circuit used with, for example, a timer, microcomputer or watch.
When power is supplied to a microcomputer, it is generally necessary for the operator to set the system at an initial stage by pushing a reset switch in order to clear the contents of a program counter or stack register. With a timer, too, it is required to define when a counter should be started in order to accurately detect an exact current point of time, and also to reset a counter once after power is supplied to the timer.
The autoclear circuit represents the type which automatically executes the initiation of the above-mentioned systems at the same time as the power supply.
FIGS. 1 and 3 denote the arrangements of the prior art autoclear circuits. FIGS. 2 and 4 represent the waveforms of signals used with the operation of the corresponding prior art autoclear circuits. The prior art autoclear circuit of FIG. 1 comprises a voltage-dividing circuit (consisting of resistors 1, 2) of power source voltage V.sub.DD and inverter 3. Namely where, with the autoclear circuit of FIG. 1, the switch 4 is closed, then the power source voltage V.sub.DD gradually rises, and is sent forth at such a level as is obtained by dividing the initial power source voltage V.sub.DD by the ratio of the value of the resistor 1 to that of the resistor 2 as measured from the voltage-dividing point a. Where the divided voltage level reaches the threshold voltage Z.sub.1 of the inverter 3, then the inverter 3 is actuated, causing the voltage of the output terminal 5 thereof which has been kept at a high level up to this point to be inverted to a low level. Sent forth from the output terminal 5, therefore, is a pulse signal indicated in a solid line A3 in FIG. 2. Application of this pulse signal as a reset signal allows for the initiation of a system. In FIG. 2, a broken line A1 represents the rise of the power source voltage V.sub.DD, and a one-dot dash line A2 denotes a voltage level occurring at the voltage-dividing point a.
The conventional autoclear circuit of FIG. 1, however, has the drawbacks that where a power source voltage V.sub.DD gently rises like that which results from the rectification of alternate current then said autoclear circuit of FIG. 1 is normally operated, but where D.C. voltage delivered from a mercury cell is applied as a power source, then said power source voltage V.sub.DD prominently rises, causing a voltage at the voltage dividing point a also to rise sharply; the inverter 3 is operated at an exceedingly high speed, causing an inverted pulse output to have an extremely narrow width; the following reset signal-receiving circuit fails to fully catch said pulse signal, preventing said autoclear circuit of FIG. 1 from being operated in a normal condition, and the power source voltage V.sub.DD is divided by the resistors 1 and 2, causing current to always flow between said source voltage V.sub.DD and earth voltage V.sub.SS and consequently resulting in large power consumption.
Description is now given of another prior art autoclear circuit of FIG. 3 in which D.C. voltage delivered from a mercury cell is used as a power source. This autoclear circuit comprises an integration circuit consisting of a resistor 6 and capacitor 7, and an inverter 8 supplied with on output from said integration circuit. Where, with the autoclear circuit of FIG. 3, a switch 9 is closed, then power source voltage V.sub.DD is impressed on said circuit, causing a signal having an integrated waveform indicated in a one-dot dash line A5 in FIG. 4 to be supplied from the integration circuit to an input point b of the inverter 8. Where a voltage signal having the integrated waveform reaches the threshold voltage Z.sub.2 of the inverter 8, this inverter 8 is operated, causing a voltage at the output terminal 10 of the inverter 8 which has been kept at the source voltage V.sub.DD up to this point to be inverted to an earth level, and consequently a reset signal A6 (solid line) to be issued. A broken line A4 waveform shown in FIG. 4 represents the rise of the source voltage V.sub. DD.
The prior art autoclear circuit of FIG. 3 also has the drawbacks that where the source voltage V.sub.DD gently rises like that of a power source resulting from the rectification of an alternate current unlike the autoclear circuit of FIG. 1, then said autoclear circuit of FIG. 3 is not actuated; namely where a time constant required for the rise of the source voltage V.sub.DD is about the same as an integration constant .tau.=CR defined by the resistor 6 and capacitor 7, then no sufficient delay takes place between the waveform A4 of a signal representing the power source V.sub.DD and the waveform A5 of a signal denoting a charge voltage at the output point b of the integration circuit, causing a reset signal to be issued with difficulty; where, with the autoclear circuit of FIG. 3, the capacitor 7 is fitted to the outside of a LSI circuit, then it is necessary to attach a connection terminal to said outside, making it necessary to use a larger chip; and even where the capacitor 7 is set in the LSI circuit, the pattern area of the capacitor 7 has to be increased in order to elevate its capacity, eventually making it necessary to apply a larger chip.